Single-crystal silicon is used for most electronic applications. Exceptions exist, such as displays and some imagers, where amorphous silicon is applied to glass substrates in order to operate the display or imager pixel. In many applications, the display or imager is fabricated on top of the silicon electronics. For application to liquid crystal displays (LCDs), amorphous silicon has provided sufficient performance. For next generation display devices such as Organic Light Emitting Diodes (OLED), Active Matrix (AM) drive transistors made from amorphous silicon have proven problematic. Fundamentally, LCDs use voltage devices, and Active Matrix Light Emitting Diodes (AM-OLED) require current devices. Attempts to extend the conventional approach involve modifying the prior-art amorphous-silicon on glass. Amorphous-silicon is applied to the entire substrate panel, typically greater than two meters on a side, and then is re-crystallized using large excimer lasers to scan a line focus across the panel. The laser has to be pulsed so as to only melt the Si surface and not the glass, as the glass melts at a lower temperature than Si. This technique results in the formation of poly-crystal silicon rather than single-crystal silicon.
The mobility of any type of amorphous or poly-crystalline transistor, including non-silicon and organic devices, is much smaller than the mobility of single-crystal silicon transistors. Electron mobility in amorphous silicon is approximately 1 cm2/V·s compared to approximately 100 cm2/V·s for poly-silicon, and approximately 1500 cm2/V·s for high-quality single-crystal silicon. It can therefore be advantageous to use single-crystal silicon in place of amorphous silicon in such devices.
Placing diodes upon a curved surface of a silicon spheroid has proved to be challenging. In the prior art, attempts have been made to lithographically define structures on spherical surfaces, but this requires non-standard optics and has had limited success. Making electrical contacts to non-planar surfaces also requires nonstandard techniques. The complexities involved in fabrication have prevented any real progress.